Rotary feeders are known in the art for metering bulk material into pneumatic conveying lines. Rotary feeders of the type to which this invention pertains typically consist of a housing having a generally cylindrical inner wall and end walls at opposite ends thereof to form a cylindrical chamber therein, an upwardly facing inlet opening for receiving material from a material holding vessel such as a storage bin for bulk material communicating with the housing chamber, and a downwardly facing outlet opening also communicating with the housing chamber and communicable with and discharging material into a high pressure conveying line such as a pneumatic conveying line or the like, where the material is conveyed by a fluid under pressure to a desired location. The rotary feeders also comprise a shaft journaled in the end walls of the housing, and a rotor mounted on or formed integrally with the shaft within the housing chamber, and a plurality of vanes projecting radially from such rotor providing a plurality of circumferentially spaced pockets adapted to receive material that falls into the pocket under the force of gravity from the inlet, to convey such material through the rotor chamber and discharge it through the outlet, where the material falls by gravity into the conveying line. Intermediate the inlet and outlet openings, the vane ends pass in close spaced relationship with the inside of the housing.
When the rotary feeder is installed in a pneumatic conveyor system, the pressure at the inlet opening can be considerably less than the pressure at the outlet opening. It is normally desirable to ensure that pressurized fluid (such as air or another gaseous fluid) cannot flow from the outlet back toward the inlet to thereby interfere with the feeding of material into the rotary feeder. It is known to provide a slot-like venting opening in the housing of the feeder before the inlet opening when seen in direction of rotation of the rotor. Through this venting opening leakage gas from the upward moving pocket that is void of bulk material is discharged. Fluid can still however, leak from the outlet into the inlet of the rotary feeder by traveling various pathways from the higher-pressure space (the space in communication with the outlet) toward and into the lower-pressure space (the space which communicates with the inlet of the housing). Such pathways may run adjacent to the inner end wall of the housing of the feeder, as there will be at least some (even extremely small) play in order to avoid jamming of the rotor shaft in the end walls in the housing and also between the vane ends and the inside of the cylinder walls of the housing. Although such leakage may not significantly effect the ability to convey many bulk materials that are charged into conveying lines, it will effect the efficient charging of materials having a lowered bulk density into a high pressurized conveying line, wherein such conveying fluid leakage may fluidize and or aerate the material to be charged when the material comes in contact with the leakage fluid at the rotary feeder inlet. Such bulk materials having a lowered bulk density include trona, hydrated lime and magnesium oxide, and it is these and other low bulk density materials that are effectively charged into high pressurized conveying lines utilizing the apparatus and method of the present invention.